A significant quantity of gold is contained in refractory ores, which contain substantial amounts of sulfide minerals such as arsenopyrite, marcasite, and pyrite. Such sulfide minerals can encapsulate significant amounts of the gold. Pressure oxidation is a common technique to unlock the encapsulated gold. In pressure oxidation, the ore or concentrate is formed into an aqueous slurry and treated in an autoclave at elevated temperature and (oxygen) pressure to oxidize the sulfur to form sulfuric acid and render the gold soluble in a cyanide lixiviant.
During subsequent cyanide leaching of the oxidized ore, preg robbing can occur. Preg robbing occurs in carbonaceous ores when natural carbon in the ore captures the liberated gold once it has been dissolved into the aqueous phase of the slurry using cyanide. Blanking agents, such as kerosene and sodium lauryl sulfate, have been used with only limited success to prevent gold adsorption onto the fine natural carbon in the ore or concentrate.
In designing an effective process for recovering gold and other precious metals from refractory, carbonaceous ores, there are a number of considerations. First, gold and precious metal recovery should be as high as possible. As little gold and precious metals as possible should be lost through sulfide encapsulation or preg robbing. Second, pressure oxidation should be autogenous if possible. In this manner, expensive heat exchangers or heaters are not required to add external heat to the autoclave feed slurry during pressure oxidation. Third, the autoclave should have as small a size as possible for a selected autoclave feed slurry capacity to minimize capital and operating costs. Finally, the oxidized autoclave feed slurry, after pressure oxidation, should be cooled as economically as possible before leaching.